Fischer-Tropsch processes for hydrocarbon synthesis from CO and H2 (syngas) are known to produce gaseous and liquid hydrocarbons as well as oxygenates which, in general, follow the well-known Anderson-Schulz-Flory product distribution.
These reactions can be carried out in fixed, fluidised or slurry bed reactors. The production of olefins and liquid fuels, especially in the gasoline range products, is most favoured by synthesis carried out in a two-phase fluidised bed reactor operating at 350° C. and 20 bar or higher pressures and usually utilising a fused alkali promoted iron catalyst. This is known as a high temperature Fischer-Tropsch (HTFT) process.
In terms of the ideal Anderson-Schulz-Flory product distribution it is clear that the C5+ selectivity has a maximum value of around 65%. In a commercial high temperature Fischer-Tropsch process performed in a fluidised bed reactor, the optimum C5+ compound yield is usually not realised, thus resulting in a much lower C5+ compound selectivity. The reason for this is that at optimum conditions for the production of maximum C5+ compounds the process is negatively influenced by other factors, one important factor being the formation of elemental carbon. The disadvantage is that the elemental carbon is deposited on the iron catalyst which causes swelling and disintegration of the particles. This powdering of the catalyst particles results in the plugging of fixed bed catalyst reactors. In a fluidised bed the fines which are produced as a result of catalyst disintegration have a high carbon content and hence have a low particle density. Because of this the fines are readily carried out of the reactors by effluent gas and will foul the downstream equipment and also the heavy oil products. Due to the swelling of the individual particles, the entire fluidised catalyst bed expands which negatively influences the reaction.
In order to reduce carbon formation one can reduce the levels of alkali promoter for the iron catalyst and one can also increase the H2:CO ratio in the syngas to be converted to hydrocarbons.
However, it is known that iron based Fischer-Tropsch catalysts with a low alkaline promoter level tend to produce light hydrocarbons and are also not easily converted to the C5+ compound range. That is, it has been found that a HTFT process with less than 0.02 mol alkali metal promoter per 100 g iron (even if a H2:carbon oxide ratio of less than 2 is used) favours products in the C1 to C4 compound range as have been published in Catalysis Science and Technology, Volume 1, 1981, pages 202-209 and WO 0197968.
The situation is accordingly that if conditions are selected (either a low alkali level or a high H2:CO ratio) to reduce formation of elemental carbon, it is expected that hydrocarbon products in the C1 to C4 range will be favoured, thus a synthesis hydrocarbon product with less than 30% by weight of C5+ product will form.
It has now been found that if hydrocarbon synthesis of syngas is carried out in the presence of a hydrocarbon synthesis catalyst under both the conditions where:    i) the hydrocarbon synthesis catalyst includes a low level of alkali metal; and    ii) the hydrogen to carbon monoxide ratio of the syngas feed stream is relatively high,formation of free elemental carbon is reduced. However it has most surprisingly been found that the use of these conditions did not result in the formation of an extremely light hydrocarbon product, (as would be expected) but unexpectedly resulted in the synthesis of hydrocarbons containing at least 30% on a mass basis C5+ compounds.